Abstract

Background

Dendrites differ from axons in patterns of growth and development, as well as in morphology.
Given that microtubules are key structural elements in cells, we assessed patterns
of microtubule stability and polymerization during hippocampal neuron development
in vitro to determine if these aspects of microtubule organization could distinguish axons
from dendrites.

Results

Quantitative ratiometric immunocytochemistry identified significant differences in
microtubule stability between axons and dendrites. Most notably, regardless of developmental
stage, there were high levels of dynamic microtubules throughout the dendritic arbor,
whereas dynamic microtubules were predominantly concentrated in the distal end of
axons. Analysis of microtubule polymerization using green fluorescent protein-tagged
EB1 showed both developmental and regional differences in microtubule polymerization
between axons and dendrites. Early in development (for example, 1 to 2 days in vitro), polymerization events were distributed equally in both the anterograde and retrograde
directions throughout the length of both axons and dendrites. As development progressed,
however, polymerization became biased, with a greater number of polymerization events
in distal than in proximal and middle regions. While polymerization occurred almost
exclusively in the anterograde direction for axons, both anterograde and retrograde
polymerization was observed in dendrites. This is in agreement with predicted differences
in microtubule polarity within these compartments, although fewer retrograde events
were observed in dendrites than expected.

Conclusion

Both immunocytochemical and live imaging analyses showed that newly formed microtubules
predominated at the distal end of axons and dendrites, suggesting a common mechanism
that incorporates increased microtubule polymerization at growing process tips. Dendrites
had more immature, dynamic microtubules throughout the entire arbor than did axons,
however. Identifying these differences in microtubule stability and polymerization
is a necessary first step toward understanding how they are developmentally regulated,
and may reveal novel mechanisms underlying neuron maturation and dendritic plasticity
that extend beyond the initial specification of polarity.